1. Field of the Invention
The present invention relates to a device and method for generating, harvesting and storing energy. The invention may be used in applications in the portable microelectronic device industry or defense industry.
2. Description of the Related Technology
Traditional energy harvesting technologies include wind turbines and solar cells. These technologies, typically characterized by a dependency on unreliable power sources and large structures, are unsuitable for providing a steady stream of replenishable energy to microelectronic technologies. These technologies are also inefficient in converting and generating power. For example, the energy conversion efficiency of crystalline silicon solar cell modules is generally below 20% and closer to 10% for flexible amorphous silicon panels. A standard solar cell harvests about 100 mW/cm2 in bright sunlight and about 100 μW/cm2 in an illuminated office. At most, solar cells have been purported to generate up to 2 watts of power.
Mechanical and vibrational energy harvesters, by comparison, efficiently generate and store a constant stream of electrical charges. Energy harvesters incorporating piezoelectric materials are particularly effective for converting mechanical energy to electrical energy and, specifically, for converting ambient motion into electrical energy.
Current piezoelectric mechanical or vibrational energy harvesters may be classified as: (1) macro-scale energy harvesters, or (2) micro-scale energy harvesters. Macro-scale energy harvesters are usually fabricated from commercial lead zirconate titanate (PZT) or piezoelectric polymers, having a thickness of at least 127 μm. Typically, these devices generate hundreds of milliwatts (mW). Because macro-scale energy harvesters require a large surface area, on the order of several tens of cm2, in order to maintain sufficient flexibility to generate power, they are generally incompatible with microelectronic devices.
U.S. Pat. No. 6,995,496 illustrates a typically macro-scale energy harvester. The patent discloses a vibration-based piezoelectric transducer capable of converting mechanical energy to electrical energy. The transducer is connected to a storage element such as a rechargeable battery or capacitor (See col. 6, line 15). In one embodiment, the transducer may be constructed from PZT having a thickness of 2 mm (See col. 7, lines 7-8). The transducer may also be constructed from PMN-PT or fine grain PZT (See col. 9, lines 38-43) and formed as a disk or sheet (See col. 9, line 51).
U.S. Pat. No. 5,801,475 also discloses a piezoelectric energy generation device capable of using piezoelectric vibrating plates and circuits to replenish voltage. The vibrating plates are connected in a circuit to a capacitor for charge collection. The device may be constructed in the form of a cantilever having a thickness of 0.5 mm (See col. 2, line 50, 54). As taught by U.S. Pat. No. 6,858,970, these piezoelectric cantilevers may be formed as an array (See abstract).
Micro-scale energy harvesters, typically fabricated from PZT thin films having a thickness of about 1 μm, produce less than 1 μW. Although these devices are of a suitable size to power microelectronic devices, they are typically unable to produce large amounts of power and suffer from energy conversion inefficiencies. Vibration-based devices typically generate only about 375 microwatts. Additionally, PZT-Al unimorphs, having Al and PZT components with respective thicknesses of 0.0024 in and 0.0105 in, were found to exhibit conversion efficiencies of about 1-4%. H. A. Sodano, G. Park, D. J. Leo, and D. J. Inman, “Use of Piezoelectric Energy Harvesting Devices for Charging Batteries,” Smart Structure and Materials: Smart Sensor Technology and Measurement Systems, Proceedings of SPIE 5050, 101-108 (2003).
An example of a micro-scale energy harvester is disclosed in British Patent application publication no. 2,326,275, which teaches a piezoelectric generator constructed from an array of piezoelectric cantilevers that may be placed on an engine. The cantilevers are constructed from thin piezoelectric films and a non-piezoelectric substrate (See Abstract). In one embodiment, the thin piezoelectric film is constructed from PZT and has a thickness of about 1 μm (See Table 1).
U.S. Pat. No. 6,737,789 issued on May 18, 2004, discloses a force-activated electrical power generator which uses a lead-magnesium titanate (PMN-PT) piezoelectric element. This patent also discloses circuitry which is connected to the piezoelectric element and outputs an electrical charge which can be stored or employed as an energy source. It appears that the preferred embodiment of this patent employs bulk, single crystal lead-magnesium titanate for fabrication of the piezoelectric element.
There currently exists a need for energy harvesting devices capable of efficiently generating and converting mechanical energy to electrical energy to produce a large and steady stream of power for microelectronic devices. The vibration-based energy harvesting device should be characterized by: (1) a high electromechanical coupling to convert the vibrations into surface charges, (2) a sufficiently thin and flexible piezoelectric cantilever to amplify the mechanical vibration and enhance energy conversion, (3) a sufficiently thick piezoelectric cantilever to maintain a large enough output voltage and hence power, and (4) a sufficiently large capacitance to generate large amounts of power and high current flow.